Frequency-related properties of SAW elements such as the center frequency for the most part exhibit a dependence on temperature, which is typically 40 ppm/K, for example. This is because as a rule if there is an increase in temperature, thermal expansion of the substrate takes place and leads to an increase of the electrode spacing in the case of interdigital transducer structures. Since this spacing determines the center frequency of the transducer and thus of the SAW element, the wavelength also increases, and the center frequency becomes lower. However, a change of the acoustic frequency is also connected with the thermal expansion since the elastic properties of the piezomaterial also change with thermal expansion. On top of that, most of the usually used piezoelectric wafer materials show significant anisotropy and have a temperature drift of properties that are dependent on the crystal axis.
In order to ensure functionality of an SAW element over a larger temperature range in spite of the temperature drift and the temperature-related drift of the center frequency, the bandwidth of the element usually has to be increased. The production of narrow-band frequency-exact temperature-independent SAW elements therefore is practically not possible with substrates like lithium tantalate or lithium niobate. However, the temperature drift is a problem for filter applications and must be minimized as much as possible.
Various measures have already been proposed to compensate the temperature drift of piezoelectric substrate materials. One possibility is to bond the piezoelectric wafer to a substrate material in a mechanically solid way and to brace it thermally. If the substrate material is appropriate selected, a certain compensation of the temperature drift can be achieved through the bracing. The compensation usually takes place so that an increase of the acoustic velocity of the acoustic surface wave used for the element is linked to the thermal expansion in the bracing material. Silicon, for example, is known as a bracing material for this.
Another possibility is to apply a dielectric layer of silicon oxide to the substrate surface and, for example, over the electrode structures, for example, by deposition from the gas phase. Depending on the properties of the layer, a temperature compensation can be achieved starting with a layer thickness of about 20 to 35% with respect to the wavelength of the SAW propagating in it. However, a disadvantage with this solution is the high stress caused by the weight of the layer and the high damping of the SAW caused by such a thick layer.